Studies of leukocytes after thermal injury have determined that significant defects occur in phagocytic cell function. This observation is explained by one of two possibilities: That after thermal injury, leukocyte growth and differentiation occurs in such a way that intrinsic defects develop preventing some vital mechanism from operating. Alternatively, factors related to the thermal injury affect the leukocyte function in a secondary manner. This proposal aims at evaluating both these possibilities by utilizing flow cytometric techniques which will allow an in- depth analysis using a model of thermal injury, the burned rat. Flow cytometry will allow investigation of multiple leukocyte functions on microliter quantities (100 ul) of whole blood. Thus, kinetic studies on single rats are now practical. While investigating this model of thermal injury, a number of new applications of flow cytometry will be applied, namely the use of the dichlorofluorescein intracellular H2O2 assay, evaluation of membrane fluidity as a means of detecting defects in early activation events and the use of a fluorescence method for simultaneously measuring phagocytosis and killing. Because of the ability of flow cytometry to utilize leukocyte preparations of mixed populations, these investigations will be performed on both the monocyte and neutrophil phagocytic cells. After development and application of the required techniques, and after the burned rat model is better understood, blood from thermally injured patients will be used. These patients will be from a special burn unit at this institution which is very keen to assist in a cooperative study. These will become significant studies in understanding the relationship between immune responsiveness and sepsis after thermal injury.